ANAEROBIC METHANE OXIDATION ON THE AMAZON SHELF

Anaerobic methane oxidation on the Amazon shelf is strongly controlled by dynamic physical sedimentation processes. Rapidly accumulating, ph ysically reworked deltaic sediments characteristic of much of the shel f typically support what appear to be low rates of steady state anaero bic methane oxidation at depths of 5-8 m below the sediment-water inte rface. Methane oxidation in these cases is responsible for < similar t o 10% of the Sigma CO2 inventory in the oxidation zone acid is limited largely by the steady-state diffusive flux of methane into the overly ing sulfate reduction zone. In contrast, a large area of the shelf has been extensively eroded, reexposing once deeply buried (>10 m) methan e-charged sediment directly to sea-water. In this nonsteady-state situ ation, methane is a major source of recently produced Sigma CO2 and an important reductant for sulfate. These observations suggest that auth igenic sedimentary carbonates derived from anaerobic methane oxidation may sometimes reflect physically enhanced nonsteady-state exposure of methane to sulfate in otherwise biogeochemically unreactive deposits. The concentration profiles of CH4, SO4=, and Sigma CO2 in the eroded deposit were reproduced by a coupled reaction-transport model. This ar ea of the shelf was reexposed to seawater approximately 5-10 years ago based on the model results and the assumption that the erosion of the deposit occurred as a single event that has now ceased. The necessary second order rate constant for anaerobic methane oxidation was greate r than or equal to 0.1 mM(-1) d(-1).